1. Field of the Invention
Embodiments of the present invention relate to an optical recording and/or reproducing apparatus and methods thereof, and more particularly, to a magnetic circuit/method using a plurality of focus coils for both focus driving and tilt driving, and an optical pickup actuator, an optical recording and/or reproducing apparatus, and methods therefore.
2. Description of the Related Art
Generally, an optical pickup is used for an optical recording and/or reproducing apparatus. The optical pickup moves over an optical information storage medium, e.g., an optical disk in a radial direction of the optical disk, and records/reproduces information onto and/or from the optical information storage medium in a non-contacting way.
The optical pickup uses an optical pickup actuator to drive an objective lens in a tracking direction, a focus direction, and/or a tilt direction to ensure light emitted from a light source forms a light spot at a correct position on the optical information storage medium. Driving in the tracking direction means adjusting the objective lens in the radial direction of the optical disk, for example, so that the light spot is formed along a center of a track.
Generally, optical pickup actuators include a lens holder movably installed at a base, a suspension supporting the lens holder to move with respect to the base, and a magnetic circuit installed facing the lens holder and the base. Such optical pickup actuators move in tracking and focus directions. With this in mind, it is noted that related optical recording and/or reproducing apparatuses are now tending to operate at high densities, with compact sizes and light weights.
To accommodate these high densities, the corresponding optical pickup actuator also needs to move in a tilt direction, in addition to the tracking and focus directions. In other words, to accommodate the high densities, the numerical aperture of the corresponding objective lens is increased and the wavelength of the respective light source is decreased. As a result, a tilt margin of the optical pickup actuator can be decreased. Thus, it is desirable for the optical pickup actuator to also move in the tilt direction, in addition to the tracking and focus directions.
In addition, to accommodate the high speeds of these optical recording and/or reproducing apparatuses, a more highly sensitive optical pickup actuator is further desired. The magnetic circuit used to obtain such a highly sensitive optical pickup actuator uses a pair of focus coils to achieve high focus sensitivity. When such a magnetic circuit is used, radial tilt driving is performed in a differential mode, where tilt driving signals having opposite phases are applied to the pair of focus coils, respectively. The differential mode thus has an advantage of high tilt sensitivity.
FIG. 1 illustrates an example of such a conventional magnetic circuit used in a conventional optical pickup actuator. This conventional magnetic circuit includes a four-pole magnet 1 having N and S-poles appropriately distributed among quadrants, first and second focus coils 3 and 5, and first and second track coils 7 and 9.
The first and second focus coils 3 and 5 and the first and second track coils 7 and 9 are installed on a side of a moving part, i.e., a lens holder, of the optical pickup actuator. The four-pole magnet 1 is installed at a base to face the focus and track coils 3, 5, 7, and 9.
As shown in FIG. 1, first through fourth magnet portions 1a, 1b, 1c, and 1d of the magnet 1 respectively corresponding to first through fourth quadrants on a y-z coordinate plane, correspond to a north pole (N-pole), a south pole (S-pole), an N-pole, and an S-pole, respectively. The first and second focus coils 3 and 5 are disposed such that the first focus coil 3 interacts with the first and fourth magnet portions 1a and 1d and the second focus coil 5 interacts with the second and third magnet portions 1b and 1c. The first and second track coils 7 and 9 are disposed such that the first track coil 7 interacts with the first and second magnet portions 1a and 1b and the second track coil 9 interacts with the third and fourth magnet portions 1c and 1d. 
In FIG. 1, hatched areas illustrate effective coil portions. In other words, hatched areas in the first and second focus coils 3 and 5 are effective focus coil portions, and hatched areas in the first and second track coils 7 and 9 are effective track coil portions.
When the above-described conventional magnetic circuit is used, a moving part of an optical pickup actuator can be driven in the focus and tracking directions and can also be driven in the tilt direction using the aforementioned differential mode.
Referring to FIG. 2A, when current flows in the first and second focus coils 3 and 5 clockwise and counterclockwise, respectively, forces act on the first and second focus coils 3 and 5 in a positive (+) focus direction (i.e., in a +z-direction). When the directions of the current flows in the respective first and second focus coils 3 and 5 are reversed, forces act on the first and second focus coils 3 and 5 in a negative (−) focus direction (i.e., in a −z-direction). Accordingly, the conventional magnetic circuit can drive an objective lens, installed in a moving part of an optical pickup actuator, in a focus direction. For focus driving, a pair of focus driving signals, for respective focus coils, having the same phase are used. Current flows in the first and second focus coils 3 and 5 in opposite directions, respectively, because the first and second focus coils 3 and 5 are typically wound in opposite directions, respectively.
When sizes of the forces respectively acting on the first and second focus coils 3 and 5, in the focus direction during the focus driving, are represented by “FA” and “FB”, respectively, the focus thrust acting during the focus driving becomes the sum of FA and FB, i.e., FA+FB.
Referring to FIG. 2B, when current is applied to the first and second focus coils 3 and 5 in the same direction (e.g., counterclockwise), a force acts on the first focus coil 3 in, for example, the negative focus direction (i.e., the −z-direction) and a force acts on the second focus coil 5 in, for example, the positive focus direction (i.e., the +z-direction). When the direction of the current applied to the first and second focus coils 3 and 5 is reversed, directions of the forces respectively acting on the first and second focus coils 3 and 5 are also reversed. Accordingly, a moving part of an optical pickup actuator can be driven in the tilt direction, e.g., in a radial tilt direction, such that a tilt of an objective lens installed at the moving part can be adjusted.
When sizes of the forces respectively acting on the first and second focus coils 3 and 5, in the focus direction while a tilt driving signal is applied, are represented by “FA′” and “FB′”, respectively, the resultant torque during tilt driving becomes R(FA′+FB′). Since the first and second focus coils 3 and 5 are used for both of the focus driving and the tilt driving, FA′=FA and FB′=FB when a magnitude of the focus driving signal is the same as that of the tilt driving signal. Here, “R” represents a distance between a central C of rotation and a central of the force acting on either of the first and second focus coils 3 and 5.
As seen from FIGS. 2A and 2B, the conventional magnetic circuit can perform the tilt driving in the differential mode using the pair of the first and second focus coils 3 and 5.
Meanwhile, when current flows in the first and second track coils 7 and 9 clockwise and counterclockwise, respectively, forces act on the first and second track coils 7 and 9 in the left direction (i.e., in a −y-direction). When the directions of the current flows in the respective first and second track coils 7 and 9 are reversed, the respective forces act on the first and second track coils 7 and 9 in the right direction (i.e., in a +y-direction).
As described above, an optical pickup actuator using the conventional magnetic circuit can drive an objective lens, installed in a moving part of the optical pickup actuator, in the focus direction. In addition, the moving part can also be driven in the tilt direction, e.g., in the radial tilt direction. Accordingly, a tilt of the objective lens installed at the moving part can be adjusted. Moreover, the moving part of the optical pickup actuator can be driven in the tracking direction so that the objective lens can be controlled to correctly follow a track. Therefore, when a pair of such conventional magnetic circuits are installed along opposite sides, respectively, of the moving part of the optical pickup actuator, the objective lens can be driven in focus directions, tracking directions, and radial tilt directions.
However, as focus sensitivity increases in the conventional magnetic circuit, tilt sensitivity also increases, which causes problems. In detail, usually in the aforementioned differential mode, the same circuit is used for both of focus control and tilt control. A pair of focus driving signals input to the circuit for the focus control have the same phase, while a pair of tilt driving signals input to the circuit for the tilt control have opposite phases, respectively. When the same circuit is used for both of the focus control and the tilt control, as described above, even when only a focus driving signal is input to the circuit with a tilt driving signal being set to zero, the tilt driving signal actually doesn't have a value of zero but has a predetermined value, e.g., a value of ±25 mV. In this situation, if the tilt sensitivity is extremely high, the tilt driving may unintentionally be provoked.
Accordingly, when the conventional magnetic circuit, which increases the tilt sensitivity when the focus sensitivity is increased, is used, undesired tilt driving may be performed while the tilt driving is not required. To solve this problem a reduction in the tilt sensitivity has been recommended. However, if the tilt sensitivity is reduced by, for example, decreasing the number of winds of the first and second focus coils 3 and 5, the focus sensitivity is also reduced. Consequently, reducing of the tilt sensitivity by decreasing the number of winds of the first and second focus coils 3 and 5 cannot be an appropriate solution.